Low potassium in the bloodstream (serum potassium levels are normal between attacks)

Weakness, especially in the muscles of the arms and legs and occasionally in the muscles of the eyes. The muscles involved in breathing and swallowing can sometimes be affected, and this can be fatal. Muscle strength is normal between attacks at first. However, repeated attacks may eventually cause worsening and persistent muscle weakness.

Between attacks, examination is normal. Or, there may be signs of hyperthyroidism, such as an enlarged thyroid.

Hyperthyroidism is confirmed by abnormal results on the following tests:

The best treatment is to quickly reduce thyroid hormone levels. Potassium should also be given during the attack, preferably by mouth. If weakness is severe, intravenous potassium may be needed. (Note: intravenous potassium should be given only if kidney function is normal and the patient is monitored in the hospital.)

Weakness that involves the muscles used for breathing or swallowing is an emergency. Patients must be taken to a hospital. Dangerous heart arrhythmias may also occur during attacks.

Your health care provider may recommend that you eat a diet low in carbohydrates and salt to prevent attacks. Medications called beta blockers may reduce the number and severity of attacks while hyperthyroidism is brought under control.

Acetazolamide is effective in preventing attacks in people with familial periodic paralysis . It is usually not effective with thyrotoxic periodic paralysis.

Expectations (prognosis):

Chronic attacks will eventually lead to muscle weakness that persists even between attacks. Thyrotoxic periodic paralysis responds well to treatment. Treating hyperthyroidism will prevent attacks and may even reverse muscle weakness.

Complications:

Difficulty breathing, speaking, or swallowing during attacks (rare)

Heart arrhythmias during attacks

Muscle weakness that gets worse over time

Calling your health care provider:

Go to the emergency room or call the local emergency number (such as 911) if you have periods of muscle weakness. This is especially important if you have a family history of periodic paralysis or thyroid disorders.

Emergency symptoms include:

Difficulty breathing, speaking, or swallowing

Fainting

Prevention:

Genetic counseling may be advised. Treating the thyroid disorder prevents attacks of weakness.